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Two towers on top of two hills are 40 km...

Two towers on top of two hills are 40 km apart. The line joining them passes 50 m above a hil halfway between the towers. What is the longest wavelength of radio waves, which can be sent between the towers without appreciable diffraction effects?

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`d sin theta = nlamda, lamda =(d sin theta)/n`
`OT_(1)=OT_(2)=20km=20xx10^(3)m`
For longest wavelength `n=1` and `sin theta~~theta~~tan theta`
`:. Lamda=(d . tan theta)/1=d.tan theta=50xx(HO)/(OT_(2))`
`=(50xx50)/(20xx10^(3))=/8m = 100/8cm=12.5cm`
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